AFB stands for "Away From Brain".
A typically AFB person is a person who acts just plainly brainlessly..AFB at first derived from the expression "afk", but "afb" stood firstly for a person , who wasn't afk, but acted just as retardedly as if he/she would've been it.
The AFB expression is used by 1337 speakers, who applies their 1337 speaking abilities to their everyday 1337'ish lifes.

Attack from behind. A pulling technique to be used in nightclubs. Approach you female target from behind and start sleasing all over her before she's even seen you. There are two variations:
UAFB - Univited attack from behind as described above.
IAFB - The girl first indicates that your advances are welcome/backs into you.